This invention relates to a method for making graft block copolymers and particularly to making saturated graft block copolymers without a hydrogenation step. More particularly, the present invention is a process for making saturated graft block copolymers from polyvinyl aromatic hydrocarbons and functionalized polyolefins.
Anionic polymerization utilizing organo alkali metal initiators has long been used to polymerize conjugated diolefins such as butadiene and isoprene and to copolymerize such diolefins with styrenes and substituted styrenes to make styrene-butadiene-styrene and styrene-isoprene-styrene linear block copolymers and similar linear block copolymers. This reaction is usually carried out in an inert hydrocarbon solvent such as cyclohexane or toluene and it is necessary to rigorously exclude oxygen, water or any impurity that can react with a highly reactive propagating species. Under these conditions, the polymeric molecular weights and block structure can be precisely controlled.
Such polymers contain a large amount of aliphatic unsaturation in the diene blocks. Since saturated polymers have certain advantages over unsaturated polymers, such as oxygen and heat stability, it has been desirable to hydrogenate the aliphatic unsaturation in the conjugated diene blocks and this has been accomplished using any of several hydrogenation processes known in the prior art. For instance, the commonly used method is to use a Group VIII metal, particularly nickel or cobalt, catalysts with a suitable reducing agent such as an aluminum alkyl to catalyze the hydrogenation of these polymers. Recently it has been found that bis(cyclopentadienyl)titanium compounds can also be used in the hydrogenation of these polymers. Excellent saturated linear block copolymers are the result of such processes.
The disadvantage in the prior method of producing saturated linear block copolymers is the necessity for the additional hydrogenation and catalyst removal steps. These steps are equipment and time intensive and thereby increase the complexity and cost of producing linear block copolymers. In addition, the hydrogenation catalysts are sensitive to certain poisons making hydrogenation of polymers containing particular functional groups or coupling agent residues difficult or impossible. It would be highly advantageous to have a process by which saturated block copolymers could be produced without the necessity of a hydrogenation step. The present invention provides such a process and also produces graft block copolymers with zero residual unsaturation. The saturated graft block copolymers of the present invention differ from the saturated linear block copolymers of the prior method in the relative location of the component polymer blocks as illustrated in the following figures. ##STR1##